1. Field of the Invention
The invention relates to test and measurement apparatus for semiconductor wafer processing systems and, more particularly, to ion energy analyzers having electrically controllable geometric filters.
2. Description of the Prior Art
Ion energy is an important parameter of a plasma contained by a reaction chamber within a semiconductor processing system. During a semiconductor etching process, ion energy affects selectivity of the etch, etch rate uniformity and residue control. Since this parameter is so important to the etch process, the measurement of ion energy at a given location within a reaction chamber is important to characterizing the effectiveness of the plasma in processing a semiconductor wafer.
Under conditions where there is directed ion bombardment perpendicular to the surface of the wafer, plasma etching typically produces anisotropic etching of the wafer surface. In addition to anisotropic etching, spontaneous chemical processes, as a result of ion enhanced chemical reactions proximate the surface of the wafer, typically result in an isotropic etching of the wafer. As such, during the plasma etching process, the ions impart energy either to the lattice structure of the wafer or to the reactant species on the surface of the wafer. The etching yield as well as etch rate uniformity are both a function of the ion bombardment angle and the energy of the bombardment ions. Thus, both ion energy and ion trajectory angle with respect to the wafer surface are important to characterizing a plasma. Consequently, to fully characterize the plasma in plasma etching processes, it is necessary to quantify both the energy and the angle distributions of ions striking the surface of a wafer during etching.
Typically, ion energy analyzers are imbedded into a support structure for the semiconductor wafer, e.g., such support structures are known as wafer chucks, susceptors, or wafer pedestals. An ion energy analyzer is a well-known device for determining the energy properties of ions within a plasma. For a detailed description of a typical gridded ion energy analyzer, see R. L. Stenzel, et al., "Novel Directional Ion Energy Analyzer", Rev. Sci. Instrum. 53(7), July 1982, pp. 1027-1031 which is hereby incorporated by reference. As described therein, a traditional gridded ion energy analyzer contains a metallic collector, a control grid, and a floating grid, all formed into a cylindrical stack where the collector and each grid are separated by a ceramic insulating washer. Specifically, the collector is a negatively biased metallic disk. The negative bias repels electrons from the collector and attracts ions to the collector. The control grid is positively biased such that ions with energies that do not exceed the positive bias are rejected by the analyzer. As such, the control grid is used to select ions for collection that have energy levels greater than a specified energy level and reject all others. The unbiased (floating) grid is a mesh screen that, being unbiased, simulates the surface of a semiconductor wafer.
In operation, the ion energy analyzer imbedded in the pedestal is either used to measure the ion energy prior to having a wafer placed upon the pedestal or a specially designed wafer having a hole to expose the energy analyzer to the plasma is placed upon the pedestal. Once the plasma is established in the chamber, ions having energies exceeding the control grid bias are collected by the collector plate and create an electrical current in an ammeter connected to the collector plate. The energy of the ions in the plasma is determined by adjusting the control grid bias and monitoring the current measured by the ammeter.
To provide fixed geometric filtering such that ions having a particular trajectory are captured by the ion energy analyzer, a micro-channel plate is provided in lieu of the floating grid in the previously described ion energy analyzer. A micro-channel plate generally contains a plurality of capillary channels formed in a fixed pattern through the plate. As such, the micro-channel plate provides depth to each passageway into the ion energy analyzer, and as such, provides ion trajectory discrimination, i.e., the thicker the plate, the more geometrically discriminating the analyzer and the narrower the angle over which incoming ions will be accepted into the analyzer.
More specifically, the plate is typically fabricated of glass, having a plurality of holes (known as micro-channels or micro-capillaries) formed in a honeycomb pattern through the plate. In particular, the plate thickness and micro-channel diameter define a critical angle measured from the long axis of a given micro-channel. Ions entering a micro-channel at a trajectory angle that is greater than the critical angle impact the walls of the micro-channel and do not enter the ion energy analyzer. On the other hand, ions with trajectory angles less than the critical angle pass into the analyzer for further discrimination, i.e., energy discrimination by the discriminator grid. In one prior art utilization of a micro-channel plate in an ion energy analyzer, each of the micro-channels had a diameter of 0.015 millimeters and a length of 0.6 millimeters which defined a critical angle of approximately 0.6 degrees. See R. L. Stenzel, et al., "Novel Directional Ion Energy Analyzer", Rev. Sci. Instrum. 53(7), July 1982, pp. 1027-1031. Of course, for cylindrical holes, the critical angle is a spherical angle. Optionally, the micro-channels can be formed at an angle (other than perpendicular) to the surface of the plate such that ions having a specific trajectory angle, plus or minus the critical angle, are geometrically selected for measurement. Furthermore, to alter the angle at which ions are accepted into the ion energy analyzer, the entire ion energy analyzer may be physically rotated to select certain ions having a specific trajectory angle.
In another prior art ion energy analyzer that determines ion energy with respect to ion trajectory angle, the control grids and the collector plate within the analyzer have a concave shape. See J. Liu, et al., "Ion Bombardment in R.F. Plasmas", J. Appl. Phys. 68(8), 15 October 1990, pp. 3916-3933. In this prior art ion energy analyzer, the collector plate is formed as a plurality of concentric conductive rings, where each ring is capable of individually measuring the energy of ions that impact a particular ring. Because of the concave shape of the collector and the control grid, ions having particular trajectory angles impact a particular collector ring. As such, a plurality of ion energy levels can be measured for each ring, and the ion energies measured at each ring are indicative of the ion energies at specific trajectory angles. Consequently, the ion energy analyzer measures a number of ion energies at specific ion trajectory angles. Thus, the analyzer determines an ion energy distribution with respect to the trajectory angles. In this prior art analyzer, the physical concavity of the collector and control grids define the critical angle.
In each of the prior art ion energy analyzers, the selection of ion angular trajectory is accomplished by physical movement of the analyzer or by physical design of the analyzer. Detrimentally, physical movement of the analyzer is hard to accurately control and can lead to anomalous trajectory measurements. Physical design of an analyzer that is capable of measuring ions at various trajectories are typically complex and costly ion energy analyzers. Therefore, a need exists in the art for an ion energy analyzer that electrically selects ions of various trajectories for measurement.